1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus and, more particularly, to a developing device included in an image forming apparatus for conveying a developer consisting of toner and carrier to a developing region while retaining it thereon, and causing the toner to deposit on a latent image electrostatically formed on an image carrier.
2. Discussion of the Background
A developing system using a developer consisting of toner and carrier, i.e., two-ingredient type developer has been customary with an electrophotographic or similar image forming apparatus due to its adaptability to high speed image formation. This kind of developing system is predominant in the field of products including copiers and laser printers. The developing system uses a nonmagnetic sleeve or similar developer carrier in which magnets are disposed. The developer carrier conveys the developer to a developing region while retaining it on its surface. In the developing region, the developer forming a brush contacts or adjoins an image carrier on which a latent image is electrostatically formed. The developer carrier is applied with an electric bias and forms an electric field between it and the image carrier, so that the toner is selectively deposited on the latent image to produce a corresponding toner image.
There is an increasing demand for high image quality on the market of image forming apparatuses of the type using the two-ingredient type developer. In this respect, image noise ascribable to the irregular deposition of the toner on the latent image is a critical problem. In a printer or a digital copier, for example, smooth halftone is not achievable unless dots are uniformly formed at intervals of several ten microns. In practice, dots are different in shape and area from each other, and the toner is irregularly deposited between clots, as observed through a microscope. An image with such irregularities appears rough and lacks uniformity.
To solve the above problem, there has been proposed to apply a bias for development including an oscillation component (oscillation bias hereinafter) to the developer carrier in, e.g., Japanese Patent Laid-Open Publication Nos. 3-67278, 4-162059, 4-356076, 7-114223, 7-333957, and 8-62955.
In the above developing system using the two-ingredient type developer, the amount of charge to deposit on the toner tends to vary as the toner content of the developer varies in dependence on the duration of agitation of the developer, toner consumption, and toner replenishment. That is, there is a trade-off between the toner content and the amount of charge. Further, image density tends to decrease with an increase in the amount of charge to deposit on the toner. This tendency is noticeable when the bias to be applied between the developer carrier and the image carrier is implemented only by a DC voltage. In light of this, the rotation speed of the developer carrier may be increased, or the gap between the developer carrier and the image carrier may be reduced, as well known in the art. However, the problem with this kind of scheme is that the carrier existing on the developer carrier scrapes off the toner image formed on the image carrier, blurring the trailing edge portion of the image or rendering lines uneven in width.
An implementation capable of increasing image density by solving the above problem and well known in the art is as follows. For the bias for development, use is made of a DC voltage on which an AC voltage is superposed. This kind of bias forms an oscillation electric field in the developing region where the developer carrier and image carrier face each other. Charged toner is caused to deposit on a latent image formed on the image carrier in the oscillation electric field. With this implementation, it is possible to activate the toner due to the oscillation of the AC voltage and to optimize frequency and peak-to-peak value which is the absolute value of a difference between the maximum value and the minimum value of the AC voltage. As a result, not only image density is increased, but also image quality is enhanced.
The prerequisite for high image quality is that the toner be efficiently deposited on the image portion of the image carrier, but prevented from depositing on the non-image portion of the same. To meet this requirement, it has recently been proposed to provide the AC voltage with a rectangular waveform and change the duty ratio of the AC voltage.
For example, Laid-Open Publication No. 7-33957 mentioned earlier discloses a developing device in which the potential of an AC voltage causing the toner to move toward the image carrier and the potential of the same causing the toner to move toward the developer carrier are fixed, and only the duty ratio of the rectangular AC voltage is varied. In the developing device, a time-average voltage is calculated from the above duty ratio and a peak-to-peak value which is the absolute value of a difference between the above two different potentials. The duty ratio of the AC voltage is controlled in order to vary the peak-to-peak value. As a result, a potential difference between the time-average voltage and the potential of the exposed portion or image portion of the image carrier is varied in order to increase image density. Further, to obviate the deposition of the carrier and the deposition of the toner on the non-image portion, i.e., fog, the peak-to-peak value is selected such that the potential causing the toner to move from the developer carrier toward the image carrier does not exceed the potential of the non-image portion or non-exposed portion of the image carrier.
Japanese Patent Laid-Open Publication No. 4-136959 teaches that the phase of the AC voltage causing the toner to move from the developer carrier toward the image carrier has a rectangular wave duty ratio of less than 50% inclusive, and that the AC voltage has a great peak-to-peak value. In this condition, a difference between the potential of the AC voltage causing the toner to move from the developer carrier toward the image carrier and the potential deposited on the image portion of the image carrier after exposure is made greater than a difference between the potential causing the toner to move from the image carrier toward the developer carrier and the potential of the image portion of the image carrier. This successfully increases image density. Further, the duration of the bias causing the toner deposited on the non-image portion to return to the developer carrier is increased in order to obviate fog. In addition, the oscillation of the AC voltage activates the toner and thereby enhances image quality.
The methods of the kind applying an oscillation bias to the developer carrier and as discussed above have a problem that they fail to improve image quality, but rather degrade it, depending on the conditions of bias application. We confirmed this problem by a series of experiments. This problem is presumably ascribable to the movement of toner and carrier which is susceptible to various factors.
Particularly, laid-Open Publications 4-356076, 7-993957 and 8-62955 describe specific conditions on frequency, peak-to-peak value and duty ratio relating to the oscillation bias having a pulse waveform. The conditions, however, do not take account of the characteristic of the developer which is caused to move by the bias, and therefore cannot insure desirable image quality with various kinds of toner and carrier.
Laid-Open Publication No. 7-114223 teaches specific ranges of particles sizes of toner and carrier and specific frequencies of the oscillation bias. This document describes that frequencies higher than 6,000 Hz inclusive are preferable, and that the carrier should preferably have a volume resistivity of 10.sup. .OMEGA.cm in order to maintain the developing efficiency. We, however, found that the above frequency range is apt to cause the trailing edge portion of a solid image to be blurred, and that the effect of the oscillation bias is practically lost when the frequency is increased limitlessly.
Although a low resistance carrier slightly improves the above point, compared to a conventional carrier having a resistance of 10.sup.12 .OMEGA.cm, it cannot realize satisfactory image quality.
When the two-ingredient type developer is used, the toner deposits on the carrier due mainly to an electrostatic force. The oscillation bias cancels the restraint of the carrier on the toner and allows it to move easily in the electric field. However, the toner is prevented from leaving the carrier when the frequency of the oscillation bias is raised, i.e., when the duration of the phase causing the toner to move in a particular direction is reduced. Conversely, when the frequency is lowered, the span of movement of the toner increases and causes the toner to deposit on the background of the image carrier. Moreover, the carrier reacts to the oscillation electric field more positively and starts moving. This results in the deposition of the carrier on the image surface or the movement of the toner deposited on the image carrier, deteriorating image quality.
An image forming apparatus with the conventional developing device has another problem that when, e.g., the composition of the developer is changed due to version-up, developing conditions matching with a new developer cannot be set.
The arrangement taught in Laid-Open Publication No. 7-333957 discussed as above is capable of improving image density. However, a series of experiments showed that the oscillation of the toner available with the AC voltage is limited by the limited peak-to-peak value of the AC voltage of the bias, deteriorating the effect of the AC voltage. As a result, the uniformity of dots is degraded, preventing high quality images from being attained.
The arrangement taught in Laid-Open Publication No. 4-136959 is disadvantageous in that because the developing conditions are maintained constant at all times, the variation of the amount of charge to deposit on the toner is apt to cause image density to fluctuate.